HFA 141b can be prepared by reacting vinylidene chloride (also designated by CV.sub.2) or 1,1,1-trichloroethane with hydrofluoric acid. These processes can be carried out either in the gaseous phase or in the liquid phase, and in the absence or in the presence of catalysts. Numerous patents relating to these manufacturing processes have been published (U.S. Pat. No. 2,894,044, EP 391,102 and EP 353,059 for gaseous phase fluorination; U.S. Pat. No. 3,833,676 for liquid phase fluorination without a catalyst; EP 361,578, EP 378,942 and EP 391,103 for liquid phase fluorination in the presence of a catalyst).
All these processes nevertheless supply an HFA 141b containing small quantities of CV.sub.2. This CV.sub.2 can be the result of incomplete conversion in the case of the CV.sub.2 +HF process, but in the case of the 1,1,1-trichloroethane+HF process it can also be the result of decomposition of the trichloroethane. In both of the two processes, CV.sub.2 can also be formed by decomposition of HFA 141b, in particular during the final treatments such as, for example, distilling, purifying and drying. Vinylidene chloride can also be present in HFA 141b in concentrations of the order of 200 ppm to 1%.
Other ethylenic impurities can also be present in HFA 141b, such as 1,1-chlorofluoroethylene, but in a much lower quantity.
CV.sub.2 and other ethylenic impurities are undesirable in large quantities in the use of HFA 141b and the specifications in regard to CV.sub.2 or in regard to the mixture CCl.sub.2 =CH.sub.2 +CFCl=CH.sub.2 have been fixed at a value less than 500 ppm.
Various processes have been proposed for carrying out the elimination of CV.sub.2 in HFA 141b. Thus, U.S. Pat. Nos. 4,940,824 and 4,950,816 describe selective absorption of CV.sub.2 on carbon-based molecular sieves and on activated charcoals; however, the absorption capacities of the two absorbents are not very large and it is difficult to be able to go down economically to a content much less than 200 ppm.
Other purifying processes have also been described, such as eliminating ethylenic impurities by photochlorination and/or by reacting with a hydracid (EP 401,493, EP 420,709 and U.S. Pat. No. 4,962,244). Although these processes are very efficient, it is difficult entirely to eliminate all traces of these ethylenic impurities (in particular of CV.sub.2), and it would be advantageous to be able to market an HFA 141b which still contains a few hundred ppm of CV.sub.2 but is nevertheless stable.
Pure HFA 141b is a perfectly stable product which undergoes no conversion or decomposition. Conversely, in the presence of certain compounds such as for example alcohols, HFA 141b can decompose; the U.S. Pat. No. 4,816,174 thus describes stabilization by nitromethane of HFA 141b/methanol mixtures. In the presence of alcohols or polyols, most chlorofluorocarbons or chlorofluorohydrocarbons also have to be stabilized, and numerous stabilizers have thus been proposed, such as epoxybutene (JP 01056630), .alpha.-methylstyrene (JP 01050829), acrylic or methacrylic esters (JP 01211538), nitromethane (JP 01128944), mixtures of nitrated derivatives and epoxides (JP 01128945), mixtures of derivatives of styrene and epoxides, phenols, acrylic or methacrylic esters (JP 01056631, JP 01056632 and JP 01211539).
The aforementioned patents in fact only relate to stabilization of chlorofluorocarbons and chlorofluorohydrocarbons during their use, most particularly during their use as blowing agents for plastic foams. To our knowledge there exists no document relating to stabilization of HFA 141b itself.
The applicant has found that an HFA 141b containing CV.sub.2 evolves slowly during storage. This instability is exhibited even for HFA 141b containing only small quantities of CV.sub.2, that is to say approximately 100 to 500 ppm or even less. The evolution of HFA 141b is in particular characterized by acidification of the product, but the formation of acid, most particularly of hydrochloric acid, is also accompanied by the formation of other products such as phosgene and peroxide products. It has been possible to demonstrate other decomposition products, in particular formaldehyde, formic acid, glyoxylic acid and monochloroacetic acid. Although the mechanism of this decomposition is not known, it is highly likely that it proceeds via peroxidation of CV.sub.2 by ambient or dissolved air and that the various detected products which have been indicated hereinabove are the result of decomposition of the peroxide. It is also impossible to know whether this possible peroxidation of CV.sub.2 concerns only the CV.sub.2 present in the HFA 141b or whether HFA 141b is not itself susceptible to decomposition into CV.sub.2 under the influence of these decomposition products of the same CV.sub.2.
This evolution of HFA 141b during storage has been observed in darkness as well as in daylight, but it is much faster in daylight; the phenomenon can be considerably speeded up by irradiation with a UV lamp.
Certain compounds such as nitromethane or nitroethane, which have been widely recommended for stabilization of chlorofluorocarbons and which have been claimed for the stabilization of HFA 141b/methanol mixtures, have absolutely no effect on the stabilization of HFA 141b containing CV.sub.2. Neither are phenolic derivatives, in particular hydroquinone monomethyl ether (HQME), very effective.
It has now been found that addition of ethylenic hydrocarbons containing at least 4 carbon atoms to an HFA 141b containing traces of CV.sub.2 allows considerable reduction, or even complete inhibition, of this decomposition reaction and that these ethylenic hydrocarbons allow very great stabilization of HFA 141b.